Mobile device screen or portion thereof with a graphical user interface



The sole FIGURE is a front view of a mobile device screen or portion thereof with a graphical user interface showing the new design.

The broken lines showing the electronic device and display screen are for the purpose of illustrating environment and form no part of the claimed design. The broken lines and slashed lines showing elements on the display screen illustrate portions of the article and form no part of the claimed design. 

CLAIM The ornamental design for a mobile device screen or portion thereof with a graphical user interface, as shown and described. 